A positive electrode terminal and negative electrode terminal (hereinafter referred to as “external terminals”) that are disposed outside a battery are electrically connected to a positive electrode and negative electrode accommodated in a battery case, respectively. An external terminal is electrically connected to an internal terminal, and the electrical connection between the internal terminal and the electrode is performed using a ribbon-shaped metal foil called a lead. Generally, one end of the lead is welded to the internal terminal, and the other end is welded to the electrode. When the lead is disconnected, charge or discharge cannot be performed. Therefore, it is important to keep the connection strength of the weld region between the internal terminal and the lead. Furthermore, when the electric resistance of the lead is high, a sufficient discharge characteristic cannot be obtained. Therefore, it is desirable to suppress the electric resistance at a low value.
Patent Literature 1 proposes that, from the viewpoint of reducing the electric resistance and keeping the connection strength, the cross section area of the lead be increased in a midway from an ultrasonic bonding portion toward the external terminal. Patent Literature 2 proposes that a plurality of electrode tabs be stacked and welded to a thick lead terminal.